This is a lesson about the definition of planet. Learners will use direct vocabulary instruction to learn the new definitions of planet, dwarf planet, and asteroid. The publicity generated by the International Astronomical Union’s definition of...(View More) Pluto as a dwarf planet in 2006 created a teachable moment for schools around the world to consider the definitions of these concepts and how discoveries necessitate a change in the language we use to talk about it.(View Less)